What do you want to do with it?
Do you want to simply have "one array"?
How about this then:

@gradient_array= ( \@x_values, \@y_values );
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You can access your pair with:

$x=$gradient_array[0]->[$index];
$y=$gradient_array[1]->[$index];
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But that's not much better than accessing two arrays, regarding memory.